Battery electrode design often has to make trade-offs between energy density and power density. Energy density is generally considered to be the amount of energy stored in a given system or region of space per unit mass. Power density is the measure of a material's ability to conduct an electric current. Typically, devices with high energy density, that is, high storage capacity, do not discharge quickly, meaning they do not have high power at the same time.
Strong demand exists for increased volumetric energy density lithium-ion (Li-ion) batteries in power supply applications. The demand arises in several places such as for long-range drivable electric vehicles (EVs), hybrid EVs and cordless electric power tools. Specifically with regard to EVs, the distance over which Li-ion powered EVs may be used is directly related to volumetric energy density. Current Li-ion batteries meet or exceed US Advanced Battery Consortium (USABC) goals for power requirements, but meet only 60% of their recommended targets for volumetric energy density. In order to increase the volumetric density of Li-ion batteries, it is important to reduce the volume of inactive components in Li-ion cells.
FIG. 1 shows a typical Li-ion cell 10, in which the active material consists of lithium-cobalt-oxide (LiCoO2) for the cathode 18 and graphite 20 for the anode part. The inactive components consist of the electrolyte, binder, carbon, separator 14, and positive and negative current collectors 12 and 16. FIG. 2 shows how Li-ion transport occurs through a portion 20 of the liquid electrolyte in FIG. 1 from anode to cathode electrode during discharging. Local depletion of ions in the liquid electrolyte 24 can occur with an electrically conductive, dense electrode. This phenomenon limits the critical current density that can result as the discharge capacity decreases when the current increases further. Thinner electrodes, approximately 100 micrometers, with shorter Li-ion diffusion length 22 have been employed in conventional Li-ion batteries to diminish this effect.
For current EV applications, large batteries are produced by stacking many layers of conventional thin electrodes. This results in a large proportion of inactive components in these batteries. Reducing the amount of expensive separators and heavy current collectors would greatly reduce expense and the amount of inactive materials present. FIG. 4 shows a Li-ion transport path 38 that using thicker electrodes such as 36 provides a direct, practical solution to increase volumetric energy density of Li-ion batteries in a manner that increases the proportion of active material to inactive material. However, FIG. 4 shows an issue with the thicker electrodes such as 36. Because of the longer diffusion paths such as 38, the electrolyte depletion increases due to poor Li-ion conductivity in complex microstructures in the diffusion paths. Current industrial fabrication processes limit the improvements that can be made to electrode architecture.